An exhaust gas treatment assembly typically comprises one or more gas permeable blocks of ceramic or mineral material which are clamped in a housing, typically cylindrical and often referred to as a can. The housing defines a flowpath with inlet and outlet connections via which exhaust gas is directed to flow through the block or blocks. Each block may act as a filter, for example, if the assembly is to function as a diesel particulate filter (DPF), or may comprise a catalyst, for example, if the assembly is to function as a diesel oxidation catalyst (DOC) or selective catalytic reductor (SCR). The blocks may be formed with numerous parallel channels separated by thin walls and opening alternately at either end of the block or otherwise as known in the art to allow gas to flow through the block from one axial end to the other.
In order to form a gas tight seal with the wall of the housing to prevent exhaust gas from bypassing the block, and in order to retain the block inside the housing against movement due to vibration or inertial forces, for example when mounted on an engine or in other mobile applications, the wall of the housing may be compressed radially inwardly, e.g. crimped or swaged, to provide a compressive force which bears radially inwardly against the block to hold it in a fixed position within the housing. Since the blocks are typically hard and brittle and may reach a high temperature in use due to the exhaust gas temperature or additional heating means, a compressible material such as a needle mat may be arranged around the block to cushion the block and accommodate differential expansion between the block and the housing. The block and mat may be arranged in the housing before compression of the housing, or alternatively may be installed in the housing after compression, for example, through a gradually tapering guide which compresses the mat as the block slides into the housing.
Since the material of the block has limited compressive strength, it is necessary to distribute the compressive force over a sufficient area of the block to provide a firm and gas tight assembly, which in turn imposes a lower limit on the axial length of the housing which must be compressed to hold the block in position. It is also necessary to provide the housing with inlet and outlet connections which can be made or unmade as required for assembly or routine maintenance, and a suitable mounting interface comprising a surface or surfaces via which the housing can be connected to a support. In some applications, for example, to avoid vibration damage when mounting the housing on an engine, it may be preferred to provide a rigid connection to minimise movement of the housing relative to the support. The overall axial length of the housing must be sufficient to accommodate the block as well as the mounting interface and inlet and outlet connections, which in turn imposes dimensional constraints on adjacent components of a vehicle or other machine on which the assembly will be used.
In practice, relatively large dimensional variations (for example, +/−2 mm or more in diameter) are known to occur in otherwise identical gas permeable blocks for use in such assemblies, even when manufactured under identical conditions in the same production facility and even in the same batch. It is therefore common to manufacture the housings as standard units with the same, predetermined dimensions, and then to radially inwardly compress the region of each housing within which the block is to be clamped, by a different radial distance to correspond to the measured dimensions of the or each individual block which is (or will be) inserted into the housing.
Since after assembly the compressed region of each housing therefore has a different diameter, it is often preferred to mount the housing on a support using a mounting strap or the like which is adjustable to conform to the dimensions of the housing. For example, US20160053661 discloses an exhaust gas treatment assembly including a heat shield and flowhood and mounted on a support via straps and saddles which are arranged to distort to conform to the dimensions of the assembly.
However, adjustable mounting means may not provide an ideally rigid connection to the support.